Just switched to a 13 mm weightlifting belt and I'm quite a bit weaker in my working sets.

My current squat 2 RM is 285lbs and 4RM is 275 lbs. This was with a 7mm weightlifting belt. Today I switched to a 13mm one and my reps were down to 275 for 1 and 225 for 5(usually a 9RM).

Just feeling a little clueless as to why that is. Any insights would really be appreciated. Thank you and have an amazing day!